This invention relates to a signal-processing circuit for a differential voltage constituting the signal to be processed, especially for a magnetoinductive flowmeter, which circuit includes at least one preamplifier and preferably an analog/digital converter connected to the output of the preamplifier.
While this invention is not limited to signal processing in conjunction with magnetoinductive flowmeters, the following description will in all aspects serve to explain the concept of the invention, its basis, its objective and the approach to attaining its objective in reference to a magnetoinductive flowmeter.
The fundamental principle of the magnetoinductive flowmeter for moving fluids goes all the way back to Faraday who in 1832 suggested that the principle of electrodynamic induction could be employed for measuring flow rates. According to Faraday""s law of induction, a moving fluid which contains charge carriers and flows through a magnetic field will produce an electrical field intensity perpendicular to the flow direction and to the magnetic field. Magnetoinductive flowmeters operate on the basis of that law whereby a magnet, typically consisting of two field coils, generates a magnetic field perpendicular to the direction of flow in the measuring tube. Within that magnetic field, each volume element of the fluid traveling through the magnetic field contributes, as a function of the field intensity building up in it, to the measuring voltage collectible via the measuring electrodes. In conventional magnetoinductive flowmeters, the measuring electrodes are configured either for direct electrical or for capacitive connection with the moving fluid.
In magnetoinductive flowmeters, the objective is to measure the voltage differential between the two measuring electrodes which is proportional to the flow rate of the fluid. In the process, unfortunately, substantial common-mode noise as well as differential aberrations of the differential voltage are encountered, stemming among others from electrochemical effects on the measuring electrodes. As a basic approach to solving this problem, the magnetic field of magnetoinductive flowmeters is periodically reversed at a certain field frequency and the differential voltage between the measuring electrodes is analyzed in frequency- and phase-selective fashion relative to this field frequency.
In a magnetoinductive flowmeter, the signal-processing circuit for the differential voltage as the signal to be processed must have, first and foremost, two capabilities needed to improve the signal-to-noise ratio and thus to simplify signal analysis. First, it must be able to substantially suppress the common-mode signal since only the differential signal is significant. Second, it must also suppress the low-frequency components of the differential signal below the measuring frequency since these low-frequency components cause relatively strong interference.
In prior art on which this invention is intended to improve, the preamplifier employed in the signal processing circuit is a differential amplifier (ref. German patents 197 16 119 and 197 16 151). The drawback there is that common-mode rejection depends on the absolute or, respectively, the relative resistance. Obtaining a high level of common-mode rejection is therefore a relatively expensive matter since it requires highly precise resistances. Besides, strong common-mode rejection is difficult to obtain over a wider temperature range.
It is therefore the objective of this invention to provide a signal-processing circuit for differential voltages as the signal to be processed, especially for use in a magnetoinductive flowmeter, which in relatively simple and cost-effective fashion permits strong suppression of both the common-mode signal and the low-frequency components in the differential signal which occur below the measuring frequency.
The signal processing circuit according to this invention which offers the solution to the problem first discussed, is basically and essentially characterized in that it employs a preamplifier that operates in the differential mode. The preamplifier preferably features a differential input and a differential output.
As mentioned above, it is important that the signal processing circuit according to this invention be capable of substantially suppressing both the common-mode signal and the low-frequency components of the differential signal occurring below the measuring frequency. Accordingly, the preamplifier in the signal processing circuit according to this invention must, in essence, constitute a high-pass filter. Given that, according to this invention, the preamplifier operates in the differential mode, the preamplifier must include two high-pass filters, one for the noninverting signal path and one for the inverting signal path.
For implementing the signal processing circuit according to this invention with a differentially operating preamplifier, the preamplifier is preferably provided with two input-coupling capacitors on the input side. It will be particularly desirable for the input impedance of the preamplifier to be higher for common-mode signals than for differential signals. The cutoff frequency for the high-pass filters, constituted of the input impedances of the preamplifier and the input-coupling capacitors, will thus be higher for common-mode signals than for differential signals.
Inherent tolerances of the two input-coupling capacitors will inevitably cause the is two signal paths to be asymmetric. This asymmetry, in turn, causes a common-mode signal to generate a differential signal. In view of the fact that, as stated above, substantial common-mode noise is to be expected, the aforementioned effect whereby the asymmetry between the two signal paths causes a common-mode signal to produce a differential signal must be suppressed as much as possible. The simplest solution would be to make the time constant large enough for the lower cutoff frequency to be as far away as possible from the measuring frequency, i.e. the field frequency. However, that would have a drawback insofar as common-mode signals and low-frequency components in the differential signal occurring below the measuring frequency could not be suppressed to the desired extent.
A preferred embodiment of the signal processing circuit according to the invention comes closer to solving the aforementioned problem in that the preamplifier is provided with two operational amplifiers and, between the inputs of the operational amplifiers and the input-coupling capacitors, a resistance network which latter includes two bootstrapped main resistances. The input resistance behind the input-coupling capacitors, which is also a determining factor in defining the time constant, is in each case the bootstrapped main resistance, with the bootstrap causing the effective input resistance to be greater than the actual value of the respective main resistance. The size of the bootstrap differs for the common-mode signals and for the differential signals, respectively.
It was pointed out at the outset that the signal processing circuit in question can incorporate an A/D (analog-to-digital) converter connected to the output of the preamplifier. The A/D converter is preferably of the type having differential inputs. Nowadays, many of the high-resolution A/D converters which are available as integrated modules offer the benefits of differential inputs. Examples include the model AD 7715 A/D converter marketed by Analog Devices, Inc. of the USA. Connecting to the output of the preamplifier an A/D converter with differential inputs provides enhanced common-mode rejection.
Another preferred embodiment of the signal processing circuit according to this invention is further characterized in that a differential-mode input amplifier is connected to the input of the preamplifier. This input amplifier makes it possible for instance to provide amplification, filtering and impedance matching between the measuring electrodes of a magnetoinductive flowmeter and the preamplifier. Moreover, the differential-mode input amplifier connected to the input of the preamplifier prevents the input-coupling capacitors from being connected directly to the measuring electrodes of a magnetoinductive flowmeter. Connecting the input-coupling capacitors directly to the measuring electrodes of a magnetoinductive flowmeter has been found to be rather undesirable.